Neptune’s museum

For most of us, the beach is our only window into the vast panoply of organisms that dwell on the floor of the ocean. Usually it affords a very meagre peephole, but on occasion special displays are mounted.

On the afternoon of July 1, as I was driving back to Auckland from the north, two thoughts seeped into my consciousness. First, could something interesting have been cast up along the coast from the violent easterly storm that had struck us a week before, and second, what was the rush?

On impulse, I turned into a reserve between Waipu and Ruakaka, a spot where Bream Bay beach is mil. a 200-metre stroll from the carpark. Wide, white empty sands and no habitation for miles. My kind of beach.

Along the high tide line was an uncharacteristically dense band of horse mussels, their fragile shells shattered by waves and the pitiless beaks of oystercatchers and gulls. Adductor muscles, tenacious beyond death, still clasped the valves together. Towards low tide mark a second patchy band of deceased mussels lay strewn across the sand, and among them a wash of other shells. These were of more poten­tial interest, but most proved to he fragments that had spent months being shuffled to and fro beneath the breakers—a temporary collection that every so often is discarded wholesale in favour of new hostages.

Most of the battered shells found cast up on beaches during normal conditions are dealt from this well-worn hand. But a good storm stirs the depths and promises more, so I headed south along the beach to investigate.

One of the seductions of beachcombing is the indistinct dark smudge glimpsed towards the water line far down the beach. Is it kelp, birds, a contour of the sand or a washup of Nep­tune’s treasures? In the thirty or so years I’ve spent fossicking, I’ve walked hundreds of kilometres pursuing such mirages. Once, six or seven kilometres down East Beach from Houhora Heads, a washup rewarded me with many unusual shellfish and sponges and a sack of kauri gum. Here at Bream Bay the material was well spread out, and of limited diversity.

There were many live ostrich feet, a filter-feeding gastropod that lives in fine sand out beyond the break­ers, and a thin scattering of bivalves, especially the attractive sunset shells which vacuum clean the subtidal sand surface with their siphons as they suck up organic matter, and a few wedge shells, cousins with similar habits. Occasional young toheroa lay among the more numerous tuatua, colourful young dog cockles and scallops, but in total there were only a few other bivalves for every gross of horse mussels.

Carnivorous gastropods proved surprisingly nutner­ous; nothing to compare with the horse mussels, but more abundant than bivalves. Plump helmet shells were not uncommon, and neither were volutes and trumpet shells. Large whelks were also stranded.

A couple of days later I visited Pakiri Beach, about 50 kilometres to the south. Here the destruction was on a much grander scale. North of the river mouth were metre-high piles of densely packed horse mussels, and among them the same suite of large gastropods that I had seen at Bream Bay, plus more. Shells which I had always considered pretty hard to come by were here in terrible abundance, all dead or dying. For one of my favourites, the helmet shell, the previous quarter century had shown me no more than 50 specimens, and I had never seen a live individual. Here there were 50 newly dead in just a few square metres of beach—thousands in total. As a teenager I had been ecstatic to discover half a dozen live individuals of the uncommon hairy trumpet with its vividly spotted animal on rocks near Huia. Here in some pockets it was so abundant that I could not avoid crunching them underfoot. The glorious large trumpet—I’d once have cheerfully traded my brother for a good specimen—was here dying by the dozens.

There was a puzzle in all this. The trumpets, in particular, are rock-dwellers, but there are no rocks within kilometres of central Bream Bay or Pakiri. Where did they live?

“Riding” the horse mussels, probably. Horse mussels sit vertically in the mud, sharp end down, buried to about two thirds of their height. The broad fan of the upper shell projects above the mud and provides a firm foothold for a number of sedentary organisms—barnacles, sea squirts, bryozoans, tube worms, perhaps the odd sponge. I suspect that the hard shore carnivorous gastropods live and feed upon this living substrate.

Further north at Te Arai Point, the scope of devastation was intermediate between that at Bream Bay and Pakiri. On the ocean beach at Mangawhai, only drifts of horse mussels were to be seen. Perhaps the other species had been repossessed by the waves already. Washups never remain on display for long.

As a one-time collector of shells, was I euphoric at this bonanza? Not at all. This was a complete debasement of values, the death of much that I had long delighted in—enjoyed for beauty, scarcity, and the pleasure of the search. This mass destruction was abhorrent. Would the wine connoisseur who savours a glass of precious vintage rejoice upon encountering twenty people splashing about in a lake of the nectar? Excess is always an enemy.

Later, at home, I decided that there remained suffi­cient life in half a dozen of the young trumpets I had picked up to warrant liberating them. Onlookers perhaps wondered about the individual wading thigh-deep into the swelling tide next morning over the rocky shelves at Manly Beach.

Over the last nine months, Aucklanders have been afforded an opportunity to view the denizens of the same reach of seafloor that I had skirted in the lower north—but without leaving city limits.

The Auckland City Council has spent $2 million on bringing sand dredged from the seafloor four kilometres off Mangawhai to improve the eroding beach at Mission Bay in the affluent eastern suburbs. Some 30,000 cubic metres of sand have been uplifted from 40 metres down, barged to Auckland and spread over the foreshore, producing the area’s finest beach.

The rejuvenated shore is not merely pleasant, it is interesting. With the sand has come an unexpected harvest of marine life, especially shells. Colourful scallops, fan shells and turrets now festoon the clean sand, and, for the patient and sharp-eyed, a wide range of uncommon species can be discovered. To date an amazing 200 species have been found sluiced out of the sand by the tide, including at least one new species and a number of rarities.

Compared to the washups described above, the Mission Bay material is very different. Scallops are numerous; horse mussels and large trumpets rare. Small shells-5 to 20 mm—are abundant on Mission Bay, whereas none were present among the large shells in the washups, suggesting that the two assemblages have come from somewhat different environments. The horse mussel community is most likely a shallow water one, found from a few metres deep out to perhaps 20 metres, in fine sandy mud, whereas the Mission Bay material comes from 40 metres down, in coarse sand.As in any museum, a shift of just a few metres can bring a whole new vista into view.